252 HEAT AND PROTOPLASM [Ch. VIII 



in hot springs at a temperature near or above that which proves 

 fatal to their close allies. 



No one doubts that in all the cases cited above the individuals 

 living in hot springs have been derived from ancestors which 

 lived in water whose temperature rarely exceeded 40° C. The 

 race has therefore become acclimatized, and the question arises : 

 How has that acclimatization been effected ? 



Now experiments have shown that organisms, when gradually- 

 accustomed thereto, may resist a temperature which would have 

 killed them if they had been suddenly subjected to it. There- 

 fore it seems probable that the acclimatization of organisms to 

 hot springs has been a slow, long-continued process, during 

 which they have become gradually accustomed to higher and 

 higher temperatures, probably attaining the hot springs by 

 slowly advancing up their effluent streams. 



This adaptation may have taken place without selection, 

 purely by the capacity of individual adaptation which organ- 

 isms possess. That individual adaptation is sufficient to account 

 for the vitality of organisms in hot springs has been shown by 

 experiment. Dxjtrochet ('37, p. 777) observed, long ago, 

 that an organism which at first seemed injured by a high 

 temperature gradually regained activity while still subjected 

 thereto. 



Thus, he found that the current of Nitella was at first diminished by 

 raising it to 27° C, but it soon became rapid again ; raised, now, to M°, the 

 circulation began to fall off again, but in a quarter of an hour, the same 

 temperature continuing, the circulation became very rapid. This phenome- 

 non was repeated, also, at 40°. Similarly, Hofmeister ('67, p. 53) brought 

 Nitella flexilis suddenly from + 18.5° to + 5°C. The streaming movements 

 ceased. After staying 15 minutes in the cooler room, however, the rotation 

 of protoplasm recovered. 



Much more important, however, are the remarkable experi- 

 ments of Dallinger ('80). He kept Flagellata in a warm 

 oven for many months. Beginning with a temperature of 

 15.6° C, he employed the first four months in raising the tem- 

 perature 5.5° ; this, however, was not necessary, since the rise 

 to 21° can be made rapidly, but for success in higher tempera- 

 tures it is best to proceed slowly from the beginning. When 

 the temperature had been raised to 23°, the organisms began 



